The present invention relates to a method of magnetic resonance imaging (MRI), in particular a method of electron spin resonance enhanced MRI, and to apparatus for the performance of such methods.
Electron spin resonance enhanced MRI, referred to herein as OMRI (Overhauser MRI) but also referred to in earlier publications as ESREMRI or PEDRI, is a form of MRI in which enhancement of the magnetic resonance signals from which the images are generated is achieved by virtue of the dynamic nuclear polarization that occurs on VHF stimulation of an ESR transition in a paramagnetic material, generally a persistent free radical, in the subject under study. Magnetic resonance signal enhancement may be by a factor of a hundred or more thus allowing OMRI images to be generated rapidly and with relatively low primary magnetic fields.
OMRI techniques have been described by several authors, notably Leunbach, Lurie, Ettinger, Grucker, Ehnholm and Sepponen, for example in EP-A-296833, EP-A-361551, WO-A-90/13047, J. Mag. Reson. 76:366-370 (1988), EP-A-302742, SMRM 9:619 (1990), SMRM 6:24 (1987), SMRM 7:1094 (1988), SMRM 8:329 (1989), U.S. Pat. No. 4719425, SMRM 8:816 (1989), Mag. Reson. Med. 14:140-147 (1990), SMRM 9:617 (1990), SMRM 9:612 (1990), SMRM 9:121 (1990), GB-A-2227095, DE-A-4042212, and GB-A-2220269.
In the basic OMRI technique, the imaging sequence involves initially irradiating a subject placed in a uniform magnetic field (the primary field B.sub.o) with radiation, usually VHF radiation, of a frequency selected to excite a narrow linewidth ESR transition in a paramagnetic enhancement agent which is in or has been administered to the subject. Dynamic nuclear polarization results in an increase in the population difference between the excited and ground nuclear spin states of the imaging nuclei, i.e. those nuclei, generally protons, which are responsible for the magnetic resonance signals. Since MR signal intensity is proportional to this population difference, the subsequent stages of each imaging sequence, performed essentially as in conventional MRI techniques, result in larger amplitude FID signals being detected.
The objective of the present invention is to provide an OMRI method in which a spatial weighting is given to stimulation of the ESR transition and hence to the FID signal enhancement. In this way the special properties of the dynamic polarization event may be used to enhance the imaging procedure, e.g. to improve signal/noise properties and reveal additional information about the subject under study, especially information on dynamic behaviour such as flow and perfusion in a three dimensional object.